In the period free from dialysis, apart from uremic substances an additional fluid volume accumulates in the dialysis patient. Apart from removing uremic substances, it is the function of the dialysis therapy to withdraw such fluid amount. This process is referred to as ultrafiltration. The fluid amount to be withdrawn is resulting from the difference of the patient's weight at the beginning of the dialysis treatment and a medically defined dry weight, namely, the weight of the patient without hyperhydration. The fluid volume of the body in which urea, the most prominent representative of small-molecular substances usually eliminated with the urine, is distributed is referred to as urea distribution volume. The urea distribution volume is approximately similar to the body fluid volume. At the beginning of treatment, this volume is composed of the “dry volume”, the fluid volume of a dialysis patient with the medically established dry weight, and the volume accumulated in the period free from dialysis. Hence it is increased by the same value as the dry weight of the patient. Thus the urea distribution volume constitutes an important parameter for monitoring the fluid state and the general condition of the patient. At present, appropriate measuring techniques are missing for determining the urea distribution volume during treatment.